Material composition and manufacturing techniques play a considerable role in reinforced or prestressed concrete behaviour under service loads. Because it is manufactured in a factory, this structural and architectural product has greater adaptability. As a precast concrete specifier or user, it’s important to be aware of the properties of the materials employed.
1. Pigments [Oxides]
Precast concrete is frequently coloured with oxide pigments, which are available in a range of colours, from dark to mild pastel tones. These can be bought in a commercially mixed form to provide a wide range of intermediate colours.
Precast concrete should be:
- Chemically inert, especially alkaline resistant
- Chemically inorganic to reduce fading caused by photochemical deterioration.
One can also use natural or synthetic pigments, although mineral oxide pigments are more popular since they meet all of the criteria.
2. Cement for Precast Concrete Structures
Cement should meet the specifications of CAN/CSA-A3000-18 to function satisfactorily in a structure. This standard classifies cementitious materials for use in precast concrete as follows:
- Blended supplementary cementitious materials
- Portland-limestone cement
- Portland cement
- Blended hydraulic cement
- Supplementary cementitious materials
There are five main types of concrete used in precast concrete structure. These are:
- Type 10 – Normal Portland Cement
- Type 20 – Moderate Portland Cement
- Type 30 – High Early strength Portland Cement
- Type 40 – Low-Heat of Hydration Portland Cement
- Type 50 – Sulphate-resistant Portland Cement
4. SCMs (Supplementary Cementitious Materials) for Precast Concrete Structures
Supplementary cementitious materials display cementitious activity when mixed with water and other chemicals. These include fly ash, powdered granulated blast-furnace slag, silica fume, and metakaolin. SCMs improve the hydration processes of ordinary portland cement (OPC) in concrete. They are either included in concrete mixes as a partial substitute to Portland cement or blended with the cement during the production process. SCMs can improve fresh concrete qualities, strengthen hardened concrete properties, and save raw material costs by supplementing the cement’s operations.
Water is another substance used in precast buildings. Ideally, it should be free of impurities that might impair the durability and strength of the concrete. Using recycled water may increase the number of alkalis and soluble salts in concrete. Always test to confirm everything meets the minimum standards. Also, make sure alkali segregates are not employed.
6. Waterproofing Admixture
When using two or more admixtures together, test them for compatibility with the cement. Consider undertaking admixture experiments with specific materials to determine the best dodge for the required performance.
For precast structures, a crystalline waterproofing admixture is an excellent choice. Carefully develop the integral crystalline admixture powder so it mixes with concrete capillary pore structures to produce a permanent waterproofing system.
Ideally, precast concrete aggregates are extremely durable. They do not decay or dissolve as a result of weather activity. Freezing, thawing, moisture content fluctuations, and temperature changes are all weathering activity factors to consider.
CSA A23.2:19 covers aggregate testing methods. Special Aggregates may be required to provide desired characteristics like colour and texture for exposed aggregate surfaces.
8. Reinforcement Bars
Carbon steel bars are often used as reinforcement material, and must adhere to CSA G30.18:21. This standard covers two types of bars:
- Type R – used for general applications
- Type W – used when restrictions in mechanical properties and chemical composition are desired. Type W bars are more ductile and more weldable than other steels. Deformed bars have the W indication rolled into the surface.
The reinforcement should be clean and devoid of hazardous materials that may impede the binding with concrete when inserted into the mold after fabrication. Such materials include retarders, grease, loose mill scale, and loose rust oil.
9. Prestressing Tendons
Steel tendons for prestressed concrete might take the form of wires, strands, or bars. For pre-tensioned members, most contractors use wires and strands. The preferred system for post-tensioned members is a 7-wire strand system. Prestressing tendons must be clean and devoid of hazardous substances such as loose mill scales, rust, dirt, oil, grease, or any other covering that could weaken the concrete and steel binding.
The choice of materials to use in a precast concrete mix significantly impacts your project in terms of meeting deadlines, the budget, and the overall quality of the end product.